This invention relates to a magnetic disk device such as a floppy disk, a hard disk, etc., used as an external storage of a computer, or the like.
FIG. 7 depicts the construction of a data read circuit in a magnetic disk device according to the prior art. In the drawing, reference numeral 1 denotes a clock synchronization circuit, 2 is a demodulation start synchronization mark detection circuit, 3 is a demodulation circuit, and 4 is an error correction circuit.
FIG. 8 shows a data field of a sector format in the conventional magnetic disk data read circuit described above. In the drawing, reference numeral 5 denotes a PLO SYNC for clock synchronization, 6 is a demodulation start synchronization mark, 7 is a data recording area, and 8 is an error correction parity byte. The data recording area 7 and the error correction parity byte 8 are modulated to 1-7 RLLC (Run Length Limited Code), 2-7 RLLC, etc.
Next, the operation of the prior art example described above will be explained. In FIGS. 7 and 8, the clock synchronization circuit 1 inputs a reproduction pulse a obtained by digitizing a reproduction signal, establishes clock synchronization by the use of the clock synchronization PLO SYNC 5, and outputs a to-be-demodulated pulse b.
The to-be-demodulated pulse b is inputted to the demodulation start synchronization mark detection circuit 2 and to the demodulation circuit 3. Detecting the demodulation start synchronization mark 6, the demodulation start synchronization mark detection circuit 2 gives a demodulation start instruction c to the demodulation circuit 3 to start demodulation. The demodulation circuit 3 outputs a demodulation data d obtained by demodulating the data recording area 7 and the error correction parity byte 8 to NRZ (Non-Return-to-Zero) and inputs it to the error correction circuit 4. The error correction circuit 4 corrects any error of the demodulation data d by the use of the error correction parity byte 8, and outputs reproduction data e of the data recording area 7.
The conventional magnetic disk device uses a specific pattern having a different frequency from that of the clock synchronization PLO SYNC 5 as the demodulation start synchronization mark 6, and utilizes complete coincidence of patterns for the detection of the demodulation start synchronization mark 6.
According to the construction described above, however, if any error exists in the detection of the demodulation start synchronization mark 6, the sector proves to be a defective sector in both of the cases where no error exists in the data recording area 7 and where an error exists in the data recording area 7 but such an error can be corrected.